Excision repair of UV- or benzo[ a]pyrene diol epoxide-induced lesions in xeroderma pigmentosum variant cells is ‘error free’

Abstract

It is known that cells from one class of xeroderma pigmentosum (XP) patients, called XP variants, carry out excision repair of UV-induced DNA damage at a normal rate and are only slightly more sensitive than normal cells to the cytotoxic effect of UV radiation, but are much more sensitive to the mutagenic effect of UV. To see if this hypermutability were the result of an ‘error-prone’, excision repair process, we irradiated fibroblasts derived from an XP variant patient, XP4BE, under conditions that allowed the cells various lengths of time for excision repair before the onset of DNA synthesis (S phase) and assayed the frequency of 6-thioguanine (TG)-resistant mutants. Cells synchronized by release from confluence (G 0 state ) and irradiated just prior to S phase showed a dose-dependent increase in mutants at very high frequencies; cells irradiated in early G 1, ≈ 12 h before the onset of S phase, showed frequencies 4 times lower. Cells irradiated in the G 0 state and allowed 24 h or 48 h for excision repair before the onset of S phase showed still lower frequencies. A comparison of the relative rates of decrease in mutant frequency with time for excision repair before the onset of S phase in XP variant cells and normal human fibroblasts after a dose of 4 or 6 J/m 2 showed that these were equal. However, for every time point, the frequency of mutants induced per dose of UV was significantly higher in the XP variant population than in the normal, suggesting that the XP variant cells have an abnormally error-prone process of replicating DNA on a template containing unexcised lesions or normal cells are by-passing many of such lesions using an error-free process. A similar comparative study in synchronized populations of XP4BE cells and normal cells, using the anti 7,8-diol-9,10-epoxide of benzo[ a]pyrene, showed that excision repair prior to the onset of S phase also decreased the frequency of mutants induced in XP variant cells by this agent. But for every dose and time point, the frequencies induced in XP4BE cells and normal cells were identical. Thus, the hypermutability of the XP4BE cells was specific to UV radiation-induced DNA lesions.